Document web3.py exceptions and hierarchy at a high level

[fselmo]

Document that all exceptions internal to web3.py inherit from a base Web3Exception class. RPC errors raise Web3RPCError and sometimes are fine-tuned to raise TransactionNotFound, BlockNotFound, TransactionIndexingInProgress, RequestTimedOut, etc...

It may be important to note that due to the nature of JSON-RPC, specifically error codes not being standardized and inconsistency in client error code implementations, it is sometimes necessary to parse error messages to raise a more fine-tuned exception - such as in the cases of TransactionIndexingInProgress and RequestTimedOut. We should document that if client teams decide to change their messaging, these exceptions may slip back to the more general Web3RPCError so that users are aware of this.